Roller applicators are well-known as such, and conventionally consist of a headpiece which screws or snaps onto the container neck. The headpiece has a part-spherical socket surface in which a plastic ball seats. The bottom and top of the ball are exposed to the liquid in the container and to the exterior respectively. In use the top of the ball is held against the skin and rolled on the skin by pulling the dispenser sideways. Liquid wetting the rear of the ball is then progressively applied to the skin from the front of the ball. The ball needs to be a reasonably close but not tight fit in its headpiece socket, so that on the one hand it can roll freely and liquid on its surface can pass through, but on the other hand the flow of liquid is limited.
There can be an issue of leakage or evaporation from the roller applicator. It is routine to provide a snap or screw cover cap to cover the headpiece and exposed ball surface when the applicator is not being used. However liquid may still leak or evaporate into the space between ball and cap and hence be wasted. With highly volatile or penetrating liquids this can be a serious problem.
It has been proposed to prevent such leakage by arranging to clamp the ball down into its headpiece socket when the dispenser is not in use. However extreme precision is needed to get an effective seal between the ball and socket surfaces, and in practice this is not achieved. It has been proposed to include an O-ring to help the seal, but these O-rings are subject to rapid wear and sealing is still poor.
It would be desirable to provide some new way of addressing the problems of leakage and/or evaporation of liquid in such a dispenser.